tirsdag 24. juli 2018

Drivkraft for fremtiden - AW&ST

Ground
tests of two different high-power electric propulsion systems are planned for
November in a key milestone in NASA’s plans for the exploration and
commercialization of space beyond Earth’s orbit.

Under
NASA’s NextSTEP program, a 50:50 cost-sharing effort,
Ad Astra Rocket and Aerojet Rocketdyne (AR) will
conduct tests in which their thrusters are planned to run for 100 hr. at a
power level of 100 kW. The tests will demonstrate electric propulsion
technology for long-duration, deep-space transportation missions.

Ad Astra will
demo its Variable Specific Impulse Magnetoplasma Rocket (Vasimr),
while the Aerojet Rocketdyne team will test
its XR-100 Nested Hall Thruster. A third contractor, MSNW, has
dropped out after failing to meet system performance goals with
its Electrodeless Lorentz Force thruster.

·High-power electric propulsion aimed at space transportation

·Ad Astra’s Vasimr can vary specific impulse and
thrust

·Aerojet Rocketdyne’s XR-100 uses nested Hall
thrusters

The
objective is to demonstrate an electric thruster generating more than 5 newtons
of thrust, with a specific impulse (Isp) of 2,000-5,000 sec., a system
efficiency greater than 60% and a specific mass less than 5 kg/kW.
Demonstrating thermal steady-state operation for 100 hr. at 100 kW will take
the two thruster designs to a technology readiness level of 5 and pave the way
for a technology demonstration mission in space.

The VX-200SS reached 100 kW during testing
in 2017, using argon as the propellant. Credit: Ad Astra Rocket

As the
three-year NextSTEP program approaches its final
demonstrations, the rival teams provided an update on progress with their
thrusters at the American Institute of Aeronautics and Astronautics’
Propulsion & Energy conference in Cincinnati in early July.

The Vasimr is
a high-power electric plasma rocket. The engine has three linked magnetic
chambers. First is the ionizer, which produces low-temperature plasma
from a neutral gas—argon has been used so far. Second is the
radio-frequency heater, or booster, which heats the plasma with radio
waves to a very high temperature. The last chamber is an open magnetic
nozzle where the heated plasma accelerates in an expanding magnetic field
to produce thrust.

The Vasimr can
vary exhaust velocity (specific impulse) and thrust without changing the
power setting of the engine. If more thrust is required, more of the
power is directed to the ionizer and less to the heater, to make more
plasma. A denser, but cooler exhaust generates more thrust.
Alternatively, by shifting more of the power to the heater, less plasma
is generated, producing less thrust, but the exhaust is faster and more
fuel-efficient.

The plasma steady-state VX-200 was
developed by Ad Astra with private funds. Credit: Ad Astra Rocket

The VX-200SS test
engine incorporates improvements made over more than three decades
of Vasimr development. These include using helicon discharge to
efficiently produce plasma in the ionizer, and ion cyclotron resonance
heating to provide a single-pass RF boost to accelerate the ions.

The next magnet
will be a high-temperature superconductor, leading to a smaller, lighter
and more thermally robust design for the first-flight engine, the company
says. Ad Astra is also moving to a quadrupole configuration,
from a dipole design, to eliminate torquing in the magnetic field. This
150-kW TC-1Q (Thruster Core-1, Quadrupole) system is aimed at a
technology demonstration mission, if funded by NASA.

X3 is installed for risk-reduction testing
in the VF5 vacuum chamber at NASA Glenn. Credit: NASA Glenn Research Center

The XR-100 electric
propulsion system is being developed
by Aerojet Rocketdyne in partnership with NASA Glenn
Research Center, the University of Michigan (UoM) and Jet Propulsion
Laboratory (JPL). It is based on the NASA/UoM-developed X3, a
three-channel 200-kW Nested Hall Thruster, combined with AR-developed
modular power processing units and mass flow controllers.

A Hall-effect
thruster traps electrons in a radial magnetic field, then uses
them to ionize propellant and accelerate the ions to produce thrust. A
neutral gas, typically xenon, is fed through the anode into an annular
channel, where the atoms are ionized by collisions with circulating
high-energy electrons. The xenon ions are then accelerated by the
electric field between the inner anode and external cathode.

Single-channel
Hall thrusters are flying at 4.5 kW, with systems up to 14 kW in design,
says AR, but to reach high power levels the X3 uses three concentric
channels. This allows increased throttling and redundancy, each channel
firing individually, all concurrently or in any combination of two. It
also provides higher power density: the 80-cm-dia. X3 producing the
same power as a 150-cm.-dia. single-channel thruster.

The passively
cooled X3 is designed to throttle from 2-200 kW, and 1,600-3,200 sec.
specific impulse when using xenon, while maintaining greater than 60%
efficiency. The full XR-100 system, to be tested in November
incorporates several design improvements, including a
third-generation JPL-developed hollow cathode to reduce erosion
for increased life and a redesigned segmented insulator ring to avoid
arcing at high temperatures.